The date (near September 22 in the northern hemisphere) when night and day are nearly of the same length and Sun crosses the celestial equator (i.e., declination 0) moving southward (in the northern hemisphere). In the southern hemisphere, the autumnal equinox corresponds to the center of the Sun crossing the celestial equator moving northward and occurs on the date of the northern vernal equinox. The autumnal equinox marks the first day of the season of autumn.
The above plots show how the date of the autumnal equinox shifts through the Gregorian calendar according to the insertion of leap years. The table below gives the universal time of the autumnal equinox. To convert to U. S. Eastern daylight saving time, subtract 4 hours, so the autumnal equinox occurs at September 23, 1998 at 1:34 a.m. EDT; September 23, 1999 at 07:23 a.m. EDT; and September 22, 2000 at 13:11 (1:11 p.m.).
Note that the times below were calculated using AutumnalEquinox[] in the Mathematica application package Scientific Astronomer, which is accurate to within only an hour or so, and in practice gives times that differ by up to 15 minutes from those computed by the U.S. Naval Observatory (which computes September 23, 1999 at 11:31 UT instead of 11:23 UT and September 22, 2000 at 17:27 UT instead of 17:11).
Date UT Date UT Date UT
09-22-1980 21:01 09-23-1990 07:06 09-22-2000 17:11
09-23-1981 02:50 09-23-1991 12:54 09-22-2001 23:00
09-23-1982 08:38 09-22-1992 18:43 09-23-2002 04:48
09-23-1983 14:27 09-23-1993 00:32 09-23-2003 10:37
09-22-1984 20:15 09-23-1994 06:20 09-22-2004 16:25
09-23-1985 02:04 09-23-1995 12:09 09-22-2005 22:14
09-23-1986 07:52 09-22-1996 17:57 09-23-2006 04:02
09-23-1987 13:41 09-22-1997 23:46 09-23-2007 09:51
09-22-1988 19:29 09-23-1998 05:34 09-22-2008 15:39
09-23-1989 01:18 09-23-1999 11:23 09-22-2009 21:28
Here is a QuickTime movie illustrating the tilt of the Earth's equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth's equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.